Things you should realize

• To move a view, you can either modify its position or offset
• if we design to move its offset, we need something to store its offset, which is a value that can store the offset's x and y position, in this case we use CGSize
• In the view you want to move, use .gesture(DragGesture) to let your view respond to your drag
• you can access the offset value caused by dragging using two modifier .onChanged and .onEnded
• The view's current offset should be equal to Firstly: the offset that's already existed before the dragging happens) + the drag offset caused by the current dragging